The overall goal of this program is to develop a strong experimental foundation for the correction of inherited diseases of bone marrow-derived cells by genetic modification of hematopoietic stem cells. Current objectives are focused on the use of recombinant oncoretroviral, lentiviral, and foamy virus vectors to achieve safe and efficient transfer of functional genes into primitive murine and human hematopoietic stem cells while maintaining their maximal hematopoietic potential. The specific aims are to 1) identify strategies to improve efficient viral transduction of HSC ex vivo while retaining their optimal homing and engrafting capabilities 2) identify non-ablative conditioning regimens and in vivo selection protocols for enhanced engraftment of transduced cells that are effective yet minimize exposure to genotoxic agents, 3) identify optimal strategies using viral -mediated gene transfer to correct the phenotype in two inherited blood disorders, X-linked chronic granulomatous disease (X-CGD) and Fanconi anemia groups A and C. Experimental approaches include the use of in vitro culture systems, NOD/SCID mouse-human xenografts, and murine models of X-CGD and Fanconi anemia previously generated by gene targeting approaches. The implementation of these aims will be shared among 3 projects and 4 core units. This proposal draws from a group of investigators with diverse but complementary experience in stem cell biology, retrovirus- and lentivirus-mediated gene transfer, molecular genetics, virology, and bone marrow transplantation. Achievement of these goals will permit the translation of this basic work to the development of clinical protocols for effective viral-mediated gene transfer of genetic blood diseases. On a broader level, these studies should provide insight into the biologic behavior of hematopoietic stem cells and the ability to manipulate them ex vivo.